1. Field of the Invention
The present invention relates to measurement of overlay between the patterns of lower and upper layers in manufacturing a semiconductor device.
2. Description of the Related Art
In manufacturing a semiconductor device, processes of forming a film to be etched, forming a photoresist, transferring a pattern from a photomask to the photoresist by photolithography, transferring the pattern from the photoresist to the film to be etched by etching, and removing the photoresist are repeated a plurality of number of times to form a circuit pattern having a plurality of layers or the like. Hence, an overlay shift between the photomasks is an important factor in determining the degree of integration of the semiconductor device.
To cope with this overlay shift, an overlay measuring pattern is formed in, e.g., a scribing region of a photomask, where neither circuit pattern nor the like necessary for the manufacture of a semiconductor device is formed, and the overlay measuring pattern is measured by an overlay measuring apparatus of an optical pattern edge detection scheme, thereby measuring the overlay accuracy between the photomasks.
FIGS. 1A and 1B show a popular related art of an overlay measuring pattern which is called a box pattern. As shown in FIG. 1A, this related art comprises an overlay measuring pattern 11 called a main scale and an overlay measuring pattern 12 called a sub scale. The overlay measuring patterns 11 and 12 are formed on photomasks for forming the patterns of lower and upper layers, respectively.
The overlay measuring pattern 11 has a rectangular portion 13 as an opening in the pattern formed from chromium or the like. The overlay measuring pattern 12 has a rectangular portion 14 as a pattern formed from chromium or the like. The overlay measuring patterns 11 and 12 have characters such as "No. 1" and "No. 2" to identify the target transfer films on which the patterns are transferred, as shown in FIG. 1A. These characters are also transferred upon transferring the patterns.
The definitions of main and sub scales are relative. The overlay measuring pattern 11 may be used as the sub scale, and the overlay measuring pattern 12 may be used as the main scale. The rectangular portion 13 may be a pattern formed from chromium or the like, and the rectangular portion 14 may be an opening in the pattern formed from chromium or the like. Alternatively, both of the rectangular portions 13 and 14 may be patterns formed from chromium or the like, or both of the rectangular portions 13 and 14 may be openings.
The overlay measuring patterns 11 and 12 are formed on the photomasks such that the centers of the rectangular portions 13 and 14 match when the photomasks are accurately overlaid, as shown in FIG. 1B. Hence, when the overlay measuring patterns 11 and 12 are overlaid, the difference between a distance x.sub.1 from a left side X.sub.1 of the rectangular portion 13 to the left side X.sub.1 of the rectangular portion 14 and a distance x.sub.2 from a right side X.sub.2 of the rectangular portion 13 to the right side X.sub.2 of the rectangular portion 14, (x.sub.1 -x.sub.2)/2, represents the overlay shift in the X direction, i.e., in the horizontal direction.
Similarly, when the overlay measuring patterns 11 and 12 are overlaid, the difference between a distance y.sub.1 from a lower side Y.sub.1 of the rectangular portion 13 to the lower side Y.sub.1 of the rectangular portion 14 and a distance y.sub.2 from an upper side Y.sub.2 of the rectangular portion 13 to the upper side Y.sub.2 of the rectangular portion 14, (y.sub.1 -y.sub.2)/2, represents the overlay shift in the Y direction, i.e., in the vertical direction.
When patterns are to be transferred to three or more target transfer films, two or more overlay measuring patterns 11 as main scales are consecutively formed on the target transfer film as the first layer, as shown in FIG. 2. Sub scales corresponding to the second and subsequent layers are overlaid on the respective overlay measuring patterns 11. Using the overlay measuring pattern 11 transferred to the target transfer film as the first layer as a reference, the overlay accuracy of the photomasks of the second and subsequent layers is measured.
A photoresist is exposed and developed, and then, heated to a temperature within the range of about 90.degree. C. to 150.degree. C. by a hot plate or oven for drying and hardening. The upper portion of the photoresist defining the opening end largely thermally shrinks due to surface tension, so the photoresist has a tapered sectional shape. Since the taper amount depends on the volume of a photoresist, the taper amount of a photoresist applied to the same thickness is proportional to its area.
As shown in FIG. 3A, of areas S.sub.1 to S.sub.4 between the sides of the rectangular portion 13 of the overlay measuring pattern 11 transferred to a positive photoresist 15 and the edges of the overlay measuring pattern 11, the area S.sub.2 of a portion in contact with the right side of the rectangular portion 13 and the area S.sub.4 of a portion in contact with the left side may be equal. However, since characters "No. 1" need be written at a portion in contact with the upper side of the rectangular portion 13, the area S.sub.1 of the portion in contact with the upper side is larger than the area S.sub.3 of a portion in contact with the lower side.
For this reason, even when taper amounts .DELTA.x.sub.1 and .DELTA.x.sub.2 of the photoresist 15 have a relation .DELTA.x.sub.1 =.DELTA.x.sub.2 in an X-direction section, as shown in FIG. 3B, taper amounts .DELTA.y.sub.1 and .DELTA.y.sub.2 of the photoresist 15 have a relation .DELTA.y.sub.1 &lt;.DELTA.y.sub.2 in a Y-direction section, as shown in FIG. 3C.
For example, when two overlay measuring patterns 11 are consecutively formed in the X direction, as shown in FIG. 4A, the taper amounts .DELTA.y.sub.1 and .DELTA.y.sub.2 of the photoresist 15 have a relation .DELTA.y.sub.1 &lt;.DELTA.y.sub.2 in a Y-direction section, as shown in FIG. 4C, and at the same time, the taper amounts .DELTA.x.sub.1 and .DELTA.x.sub.2 of the photoresist 15 also have a relation .DELTA.x.sub.1 &lt;.DELTA.x.sub.2 in an X-direction, as shown in FIG. 4B.
When the taper amount of the sectional shape of the photoresist 15 on a film 16 to be etched is nonuniform, as shown FIG. 5A, the retreat amount of the etched film 16 increases at the largely tapered portion of the photoresist 15, as shown in FIG. 5B. For this reason, as shown in FIG. 5C, the overlay measuring pattern 11 is transferred to the etched film 16 while the positions of sides of the rectangular portion 13 shift from the original transfer positions.
Consequently, when the next film 17 to be etched is formed on the etched film 16, and the rectangular portion 14 of the overlay measuring pattern 12 is transferred to a photoresist 18 on the film 17 to be etched, as shown in FIG. 6A, a measurement error occurs as if the rectangular portion 14 shifted in both the X and Y directions, although the rectangular portion 14 is accurately formed at the original position, as can be seen from FIG. 6B.
Reportedly, when a 1.2-.mu.m thick photoresist is heated at 120.degree. C. for 90 sec, the above-described measurement error reaches 0.1 .mu.m. Hence, erroneous information may be fed back to the next lot, or the photoresist 18 is reformed, resulting a decrease in yield.